Turn `pyrakoon.client` into a package
Move client utility functions into a proper module
Split-up `Client` into `AbstractClient` and `ClientMixin`
Don't mixin `ClientMixin` into `tx.ArakoonProtocol`

It's up to API users to mix `AbstractClient` implementations with any
client-call mixins they want.
Add administrative client mixin
Don't add `ClientMixin` to `SocketClient` by default